Convicted Boston Marathon bomber Dzhokhar Tsarnaev has been sentenced to death over the April 2013 attack that killed three people and wounded 264 others.

A Boston jury, consisting of seven men and five women, reached the decision Friday in the penalty phase of the trial after deliberating for more than 14 hours over three days.

The jury unanimously determined that, based on the "aggravating and mitigating circumstances," Tsarnaev should be sentenced to death.

Karen Snyder, right, and Kathryn Vanwie react to the announcement of the death penalty verdict for Dzhokhar Tsarnaev outside the John Joseph Moakley United States Courthouse in Boston, May 15, 2015.

Unanimous decision

The jury had to reach a unanimous decision for Tsarnaev to receive the death penatly. Otherwise, he would have spent the rest of his life in prison without parole.

VOA's Fatima Tlisova reported from the courtroom that some of the jurors were crying as Tsarnaev sat with his head down.

In a statement following the verdict, U.S. Attorney General Loretta Lynch accused Tsarnaev of "coldly and callously" perpetrating a terrorist attack. She said no verdict can "heal the souls of those who lost loved ones, nor the minds and bodies of those who suffered life-changing injuries."

But she said the death penalty is a "fitting punishment for this horrific crime" and said she hoped the case's completion brings closure to the victims and their families.

U.S. Attorney Carmen Ortiz, who prosecuted the case, said at a news conference that the verdict was "fair and just." She said Tsarnaev believed in an ideology of hate and expressed that ideology by killing and maiming Americans, noting that he will "pay with his life."

Vincent Lisi, the FBI special agent in charge of the case, said the bombing victims motivated and inspired the investigators every day.

Tsarnaev, 21, was found guilty last month of all 30 terrorism and other charges. Of those, 17 carried the possibility of the death penalty.

Jurors' deliberations

Jurors were required to fill out a lengthy, complicated verdict form that asked them to make findings on 12 aggravating factors prosecutors said support a death sentence and 21 mitigating factors Tsarnaev's lawyers said instead support a sentence of life in prison.

The jurors then had to weigh any mitigating factors against any aggravating ones to determine Tsarnaev's sentence.

During the penalty phase of the judicial proceedings, prosecutors brushed off defense arguments that Tsarnaev was under the strong influence of his older, radicalized brother, Tamerlan, who was killed in a police shootout.

They said even though he was only 19 at the time of the marathon bombing, Dzhokhar Tsarnaev was old enough to know right from wrong, and he wanted to avenge U.S. wars in Iran and Afghanistan.

The Boston Marathon bombing was one of the bloodiest terrorist attacks in the United States since September 11, 2001.

Two pressure-cooker bombs packed with shrapnel exploded near the marathon finish line on April 15, 2013. The Tsarnaev brothers also killed a Massachusetts Institute of Technology police officer days later.

Mike Eckel and Mike Richman contributed to this report from Washington. Some material came from The Associated Press.